1. Field of the Invention
The present invention relates to a communication system in which a soft handover process and a multicast communication process are carried out in communication between a mobile terminal and a communication terminal device, a multicast switching device, and a communication method.
2. Description of the Related Art
Conventionally, there has been available a universal mobile telecommunication system (UMTS) as a mobile communication system standardized by 3rd generation partner project (3GPP). In this UMTS, a W-CDMA system is used as a multiplexing system of a radio communication path. In the UMTS, a soft handover system (diversity handover system) is used as a handover system of a mobile terminal. In the soft handover system, a subscriber line extension system is used. In the UMTS, for example, when data communication is carried out between a communication terminal device and the mobile terminal, a device which carries out multicast communication (device present in a communication path between the mobile terminal and the communication terminal device, referred to as a multipath point device hereinafter) is not changed while a soft handover process is being executed by the mobile terminal. That is, in the UMTS, even if the mobile terminal moves and switches base stations to communicate with in radio communication, the following processing is carried out. That is, data sent from the communication terminal device is always transmitted to the mobile terminal through the multipath point device.
The UMTS includes a core network section (CN section) and a radio access network section (RAN section). In the UMTS, it is a radio network controller (hereinafter, referred to as RNC), arranged in the RAN section, that becomes the multipath point device. When the UMTS is used to carry out data communication between one mobile terminal and one communication terminal device, the number of multipath point devices is one (see 3GTR 25.832 “Manifestations of Handover and SRNC Relocation”).
An example of processing carried out by a communication system using such a UMTS is as follows. FIGS. 1 and 2 are views for explaining the processing carried out by the communication system. The communication system includes a communication terminal device 210, a mobile subscriber switching device (MSC or SGSN) 220 connected to the communication terminal device 210 through a public network (not shown), a plurality of RNCs 231, 232 connected to the mobile subscriber switching device 220, a plurality of base stations 241, 242, . . . connected to the RNCs (in star connection), and a mobile terminal 250 which carries out radio communication with the plurality of base stations.
In the communication system shown in FIGS. 1 and 2, a soft handover process is carried out by the mobile terminal 250, and the mobile terminal 250 is in a state of being capable of executing radio communication with the plurality of base stations 242, 243. Here, a network including the mobile subscriber switching device 220 corresponds to the CN section. Note that the CN section may include a mobile gateway switching device (not shown), which is arranged between the mobile subscriber switching device 220 and the communication terminal device 210 in the communication system. A network including the RNCs 231, 232, and the base stations 241, 242, corresponds to the RAN section.
Each of the RNCs 231, 232, carries out a control process for radio resources and a control process necessary when the mobile terminal 250 moves and executes a handover process. The mobile terminal 250 carries out a well-known maximum ratio combining process based on data sent from a plurality of base stations. Thus, the mobile terminal 250 obtains data of high communication quality.
Operation of this communication system is as follows. First, when the mobile terminal 250 starts data communication with the communication terminal device 210, the RNC 231 is determined as a multipath point device. In the data communication between the mobile terminal 250 and the communication terminal device 210, the RNC 231 is always a multipath point device (anchor multipath point device). Hereinafter, this device (RNC 231) will be referred to as a serving RNC (SRNC). As shown in FIG. 1, when the mobile terminal adds or switches over to another radio communication opponent, which is a base station connected to the SRNC 231, (the case will be referred to as Intra-RNC handover hereinafter), the SRNC 231 transmits data sent from the communication terminal device 210 to the plurality of base stations 242, 243 (base stations to execute radio communication with the mobile terminal 250) in a multicast manner. Data is then transmitted from each of the base stations 242, 243 to the mobile terminal 250 (communication paths A-1, A-2).
On the other hand, when the mobile terminal 250 moves and therefore executes radio communication with the base stations 242, 243 connected to the SRNC 231, and at the same time, executes new radio communication with the base station 244 connected to the RNC 232 different from the SRNC 231 (referred to as Inter-RNC handover), the following processing is carried out. That is, as shown in FIG. 2, when data is sent from the communication terminal device 210 to the mobile terminal 250 through the plurality of base stations 242, 243, 244, the data sent from the communication terminal device 210 is transmitted to the SRNC 231 through the mobile subscriber switching device 220. The SRNC 231 then transmits the data to the plurality of base stations 242, 243 in a multicast manner. Subsequently, the data is sent from each of the base stations 242, 243 to the mobile terminal 250 (communication paths B-1, B-2).
On the other hand, when data is sent from the communication terminal device 210 to the mobile terminal 250 through the base station 244, it is only the SRNC 231 that can execute multicast transmission. Accordingly, the data sent from the communication terminal device 210 is once transmitted to the SRNC 231 through the mobile subscriber switching device 220 (communication path B-31). The SRNC 231 then transmits the data to the RNC 232 (communication path B-32). The RNC 232 transmits the data to the base station 244 (communication path B-33). The base station 244 transmits the data to the mobile terminal 250.
Additionally, when data is transmitted from the mobile terminal 250 to the communication terminal device 210, the following processing is carried out. In the case of Intra-RNC handover, the SRNC 231 transmits data sent from the plurality of base stations 242, 243 to the mobile subscriber switching device 220. The mobile subscriber switching device 220 transmits the data to the communication terminal device 210.
In the case of Inter-RNC handover, the following processing is carried out. That is, when the mobile terminal 250 sends data to the communication terminal device 210 through a base station, first, the data sent from the mobile terminal 250 is transmitted to the plurality of base stations 242, 243. The SRNC 231 obtains the data transmitted from the plurality of base stations 242, 243, and sends the data to the mobile subscriber switching device 220. The mobile subscriber switching device 220 transmits the data to the communication terminal device 210.
On the other hand, when the mobile terminal 250 sends data to the communication terminal device 210 through the base station 244, the data sent from the mobile terminal 250 is transmitted to the base station 244. The base station 244 transmits the data to the RNC 232. The RNC 232 transmits the data to the SRNC 231 through the mobile subscriber switching device 220. The SRNC 231 transmits the data to the mobile subscriber switching device 220. The mobile subscriber switching device 220 transmits the data to the communication terminal device 210.
In such an event, it is the SRNC 231 that executes various control processes concerning the data communication between the communication terminal device 210 and the mobile terminal 250. The RNC 232 carries out only a data relaying process (see 3G TR 25.832 “Manifestations of Handover and SRNC Relocation”).
However, the following problems have been inherent in the foregoing prior art. In the case of the Inter-RNC handover described above, when the data is sent from the communication terminal device 210 to the mobile terminal 250 through the base station 244, the data is transmitted to the mobile terminal 250 through the mobile subscriber switching device 220, the SRNC 231, the RNC 232, and the base station 244.
Here, the data is transmitted from the mobile subscriber switching device 220 to the RNC 232 through the SRNC 231. Consequently, in the foregoing prior art, compared with a case in which data is directly transmitted from the mobile subscriber switching device 220 to the RNC 232, the path of the data communication from the communication terminal device 210 to the mobile terminal 250 includes a redundant path.
Similarly, in the case of the Inter-RNC handover, when the data is sent from the mobile terminal 250 to the communication terminal device 210 through the base station 244, the data is transmitted to the communication terminal device 210 through the base station 244, the RNC 232, the SRNC 231 and the mobile subscriber switching device 220.
Here, the data is not transmitted directly from the RNC 232 to the communication terminal device 210 through the mobile subscriber switching device 220. Consequently, in the foregoing prior art, compared with a case in which data is directly transmitted from the RNC 232 to the communication terminal device 210 through the mobile subscriber switching device 220, the path of the data communication from the mobile terminal 250 to the communication terminal device 210 includes a redundant path.
The presence of such redundant paths has caused a problem of wasteful consumption of network resources in the data communication between the communication terminal device 210 and the mobile terminal 250.
In order to solve this problem, the following processing need be carried out in the communication system. That is, not only the SRNC 231 but also the mobile subscriber switching device 220 need be capable of transmitting data sent from the communication terminal device 210 to a plurality of devices in a multicast manner, or the mobile subscriber-switching device 220 need be capable of transmitting the data sent from a plurality of devices (including devices other than the SRNC) directly to the communication terminal device 210.
Therefore, in the case of radio communication executed by the mobile terminal 250 with the plurality of base stations (in soft handover state), it is possible to prevent wasteful consumption of network resources if each of switching devices can carry out processes (e.g. multicast communication process) of the multipath point device during the data communication between the mobile terminal 250 and the communication terminal device 210.
Furthermore, data sent from the communication terminal device 210 to the mobile terminal 250 is transmitted to the plurality of base stations 242, 243, 244. In order to realize soft handover, the data sent from each of the base stations 242, 243, 244 need be received by the mobile terminal 250 at the same timing. Additionally, when the data sent from the mobile terminal 250 is transmitted to the communication terminal device 210, communication quality of the data obtained by the communication terminal device 210 need be made good.